Transcript Optical studies in the MCM-48 meso

Optical studies of
meso-porous siliceous
Y. J. Lee a,c
J. L. Shen b,c
a Department of Computer Science and Information Engineering,
Tung Nan Institute of Technology, Taipei, Taiwan, R.O. C.
b Department of Chemistry, Chung Yuan Christian University,
Chung-Li, Taiwan, R.O.C
c Center for Nanotechnology, CYCU, Chung-Li, Taiwan, R.O.C
Introduction
The scientists of the Mobil Oil
company firstly synthesized
M41S-type meso-porous
materials, such as
MCM-41and MCM-48 in 1992.
(MCM:Mobil Composition of Matter n.)
The simulate synthesis process of MCM-41
MCM-41 has hexagonal
arrangement of
unidirectional pores with
very narrow pore size
distribution, which can be
systematically varied in
size from approximately
~20 to 200Å.
http://terra.cm.kyushuu.ac.jp/lab/
research/nano/Quantum.html
The simulate model image of
MCM -41and MCM-48
(a) MCM-41 has
hexagonal
arrangement of
unidirectional pores
(b) MCM-48 has a
cubic structure,
gyroid minimal
surface.
www.ill.fr/AR-99/page/ 34liquids.htm
Introduction
There have been few reports on the
optical properties of MCM-41 and
MCM-48.
The optical properties are not only
offer a convenient way to clarify the
structural defects, but also provide
useful information for extending
their applications to optical devices.
Experiment
The photoluminescence (PL) spectra were
taken by using a focused Ar+ laser (488nm)
and He-Cd laser (325nm) at room
temperature.
The Time-resolved Photoluminescence (TRPL)
spectra were measured with temperature
dependence and using a solid-state laser
(396 nm) with a pulse duration 50 ps as the
excitation source.
The MCM-41 and MCM-48 samples were
subjected to rapid thermal annealing (RTA)
at 200 ℃,400℃,600℃,800℃ in N2 gas
atmosphere for 30 sec, respectively.
Experiment
Monochromator
Notch filter
Laser
line filter
Polarizer
396 nm pulse laser
Raman
measurement
Polarization
of photoluminescence
Photoluminescence
measurement
Experiment
IRTA
Non-porous
Microporous
Macoporous
(>50nm)
(<2nm)
Mesoporous
Macoporous
Six characteristic shapes of the physisoption isotherms.
[K. S. W. Sing et al. Pure. Appl. Chem .57 (1985) 603]
The adsorption mechanism is controlled by the
characterization of microporous and mesoporous materials.
The Profile of MCM-41
800
absorption
deabsorption
700
3
Volume Adsorbed (cm /g)
Intensity (a. u.)
(100)
(110) (200)
(210)
1
2
3
4
5
2£c(deg.)
X-ray diffraction pattern of
siliceous MCM-41 nanotubes.
6
600
500
3.3 nm
400
300
200
100
2
3
4
5
Pore diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Isotherms of N2 adsorption on
siliceous MCM-41 nanotubes.
The inset shows the pore-size
distribution curve.
PL Intensity (a.u.)
Result and Discussion
2
MCM-41
1
MCM-48
1.4
1.6
2
1
1.8
2.0
2.2
2.4
2.6
Energy (eV)
PL spectrum of as-synthesized MCM-41 and MCM-48 at room
temperature. The dashed lines are fitted Gaussian components
Hydrogen bonded
silano groups
H
H
O
Si
Single silanol
group
H
O
Si
H
O
Si
O OO
H
O
Si
O
Si
SiO2
surface
NBOHC
H
hν
O
‧
‧
O
+ H
Si
Si
O OO
O OO
Single silanol group
Result and Discussion
800¢J
800¢J
MCM-48
PL Intensity (a. u.)
MCM-41
600¢J
600¢J
400¢J
400¢J
200¢J
200¢J
UnRta
UnRta
1.6
2.0
Energy (eV)
2.4
1.6
2.0
2.4
Energy (eV)
Photoluminescence spectra of MCM-41 and MCM-48
after RTA at room temperature.
The hydrogen-bonded silanol groups are dehydroxylated due to water
removing and form siloxane bonds and single silanol groups.
The dehydroxylation of hydrogen-bonded silanol groups take place to form
single silanol groups, leading to the generation of NBOHCs and the increase
of the PL intensity of MCM-41 and MCM-48 simultaneously.
(strain siloxane bridge)
As TRTA increases further (TRTA＞ 400 oC), the single silanol groups
with longer distance can then be dehydroxylated and give rise to the
formation of the strained siloxane bridges.
1.
2.
Strained siloxane bridge has been demonstrated to create NBOHCs
and surface E’ centers (i.e.,≡Si•)
We suggest that the 2.16-eV PL origins from the NBOHCs
associated with the strained siloxane bridges.
[ D. L. Griscom and M. Mizuguchi, J. Non-Cryst. Solids 239 (1998) 66 ]
1.0
PL Intensity (a. u.)
PL Intensity (Normalized)
Result and Discussion
Red
0.8
0
200
400
600
Time (sec)
0.6
MCM-41
0.4
0.2
MCM-48
0
1000
2000
3000
Time (sec)
PL degradation of MCM-41 and MCM-48 as a function of irradiation time.
The inset plots MCM-48 PL degradation as a function of irradiation time,
including a dark period (without laser irradiation).
PL Intensity (Normalized)
Result and Discussion
1.0
0.8
0.6
Vacuum
H2
Air
0.4
0.2
O2
0.0
0
500
1000 1500 2000 2500 3000 3500
Time (sec)
PL degradation of MCM-48 as a function of irradiation time
1.0
MCM-41
0.8
0.6
Air
0.4
O2
0.2
0
1000
2000
Time (sec)
3000
PL Intensity (Normalized)
PL Intensity (Normalized)
Result and Discussion
1.0
MCM-48
0.8
0.6
0.4
Air
0.2
O2
0.0
0
1000
2000
Time (sec)
The Red-PL degradation of MCM-41 and MCM-48 as a
function of irradiation time in air and O2 ambient gases.
3000
PL Intensity (Normalized)
Result and Discussion
1.0

e  O2  O
0.8
－
2
0.6
2.25 eV
Degassing on
0.4
0.2
0.0
0
2000
4000
6000
8000
10000 12000
Time (sec)
Evolution of PL intensity of MCM-48 as a function of irradiation time in O2 gas.
we suggest that O2－ molecules can recombine with NBOHC
on the surface, leading to the quenching of NBOHCs
PL Intensity (a.u.)
Result and Discussion
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
Energy (eV)
PL spectrum of MCM-41 at room temperature.
Result and Discussion
PL Intensity (a.u.)
800¢J
600¢J
400¢J
200¢J
UnRTA
1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4
Energy (eV)
Photoluminescence spectra of MCM-41
after RTA at room temperature.
(strain siloxane bridge)
E’ centers
NBOHCs
Both surface E’ centers and NBOHCs increase after the RTA treatment with TRTA＞ 400 oC
The surface E’ centers can combine and produce the twofold-coordinated silicon
centers, which emits the blue-green luminescence in the triplet-to-singlet transition
Result and Discussion
B. L. Zhang et al.
The first-principles
calculations.
The T1→ S0 is about 2.5 eV,
is in agreement with our
experimental result.
B. L. Zhang and K. Raghavachari, Phys. Rev. B 55, R15993 (1997)]
Intensity (a.u.)
Results and Discussion
2.6
2.8
3.0
3.2
3.4
3.6
3.8
4.0
Energy (eV)
PLE spectrum of the 2.5-eV emission band from MCM-41.
Result and Discussion
PL Intensity (a.u.)
I

P
I
 I 
 I 
I
I
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
Energy (eV)
Polarized PL spectra of MCM-41 nanotubes
Result and Discussion
The PLE measurement:
The value for the direct singlet-to-triplet excitation transition in
two-coordinated Si is around 3.3 eV
[L. Skuja J. Non-Cryst. Solids 149, 77 (1992)]
[ G. Pacchioni and G. Ierano, J. Non-Cryst. Solids 216, 1 (1997) ]
The Polarized PL spectra:
The degree of polarization P of 2.5 eV calculated was found to be
0.25, which agrees well with the P value (0.22) obtained from the
reported triplet-to-singlet transition in twofold-coordinated silicon
[L. Skuja, A. N. Streletsky, and A. B. Pakovich Solid State Commun. 50, 1069 (1984)]
Time-resolved Photoluminescence (TRPL)
Detector
Sample
Lens
Lens
Lens
Pulse Laser
Mirror
Result and Discussion
2.5 eV PL Intensity (a. u.)

Theory fit : I (t )  I 0 exp -  t   


(b) 40K
(a) 15K
0
4
8
12
16
0
4
8
4
8
12
Time (nsec)
16
16
(d) 300K
(c) 100K
0
12
0
4
8
12
16
Time (nsec)
The photoluminescence decay profile of MCM-41
at different temperatures.
Time Constant (nsec)
Result and Discussion
2.9
   
1
2.8
2.7
2.6
2.5
Time Constant
Ea=28 meV
2.4
2.3
2.2

1
nr
1
r
1
nr
Ea
 exp( )
kT
2.1
2.0
0.00
0.02
0.04
-1
1/T(K )
0.06
 r : radiative recombination time
 nr : nonradiative recombination time
Temperature dependence of the recombination time constant
Result and Discussion
Intensity (a.u.)
£G=30meV
MCM-41 Raman
396 nm
0
200
400
600
800
1000
-1
Wavenumber (cm )
Raman spectra of MCM-41 nanotubes
(nonbridge oxygen atom)
Result and Discussion
Y. Kanemitsu attributed the active energy Ea to the
phonon-related processes in the inhomogeneous
surface of the oxidize Si nanocrystals.
For nonradiative recombination process, they suggested
that the carriers undergo the phonon-assisted tunneling
from the radiative recombination centers to the
nonradiative centers
Y. Kanemitsu, Phys. Rev. B 53, 13515 (1996)
40K
PL Intensity (a. u.)
PL Intensity (a. u.)
Result and Discussion
12K
100K
200K
300K
0
50 100 150 200 250 300 350
1.6 1.8
2.0 2.2 2.4 (K)
2.6 2.8 3.0
Temperature
Energy (eV)
The variation of the luminescence intensity
with temperature of the MCM-41.
Result and Discussion
At low temperatures:
The PL intensity reaches
only phonon emission
the maximum value at
At
temperatures:
40high
K, implies
that the
phonon
absorption
radiative
transitionbecome
is
dominant
pronounced and fast
enough
to overcome the
The
phonon-assisted
nonradiative
escapethe
due
transition
dominates
to the small activation
recombination
process at
energy
in radiativeand the
high
temperatures,
transition
(Δ).of PL decay
time
constant
and the PL intensity
decreases.
Conclusion
Two PL bands were observed at around 1.9 eV
and 2.15 eV ,which can be explained by the
surface chemistry in MCM-41 and MCM-48.
The around 1.9 eV is assigned to the NBOHCs
and the around 2.15 eV is related to the
NBOHCs associated with the strained siloxance
bridges.
The PL intensity can be enhanced by the RTA
treatment.
We suggest the PL degradation origins from
the recombination of O2－ and NBOHC.
Published in Solid State Comm. 122, 65 (2002)
Micrpor. Mespor. Mater. 64, 135 (2003)
The blue-green PL in MCM-41 and MCM-48 were
attributed to the twofold-coordinated silicon
centers, which emit luminescence by the tripletto-singlet transition.
The PL intensity can be enhanced by the RTA
treatment with increased the concentration of the
surface E’ center.
We consider the PL decay dynamics with
temperature dependence by TRPL measurement
and depict that the nonradiative process, which is
associated with the phonon-assisted transition,
dominates the recombination mechanism at high
temperatures.
Published in J. Phys-condens. Mater. 15, L297 (2003)